1. Field of the Invention
The present invention relates generally to a luminaire with improved light utilization efficiency, more particularly, to a polarized luminaire, an unpolarized luminaire, an illuminance distribution improving device therefore, and polarization converter each for use in a liquid crystal projection device.
2. Description of the Related Art
In projectors such as a liquid crystal projector and a movie projector, there has been a requirement for increasing light utilization efficiency of light emitted from a light source in order to obtain a brighter projected image.
FIG. 47 shows a schematic sectional view of a prior art luminaire taken along the optical axis thereof, the luminaire being employed in such projectors.
In the figure, the arc gap center of a metal halide lamp 11 coincides with a focal point of a parabolic mirror 12, and light emitted from the metal halide lamp 11 is reflected by the parabolic mirror 12 so as to be substantially in parallel light beams. The parallel beams enter into a lens array 21. A lens array 22 having lenses arranged in a corresponding manner to those of the lens array 21 is disposed opposite thereto. A distance between the lens arrays 21 and 22 is made equal to a focal length of the lenses of the lens array 21 so that light beams emitted from the lenses of the lens array 21 enter into the corresponding lenses of the lens array 22 efficiently. A condenser lens 23 is disposed on the lens array 22 side and between the lens array 22 and a plane 24 to be illuminated.
Parallel luminous flux incoming to any lens 211 of the lens array 21 converge at the center of a lens 221, corresponding to the lens 211, of the lens array 22, pass through the condenser lens 23 and proceed to the plane 24 to be illuminated so as to illuminate the entire area thereof. That is, all images of the lenses of the lens array 21 are formed on the plane 24 in overlapping by the lens array 22 and the condenser lens 23. An intensity distribution of light reflected from the parabolic mirror 12 has the highest in the central and is lower with going apart from the center, but if the number of lens pairs of the lens arrays is large (generally the number ranges from tens to hundreds), incident light intensities all over the plane 24 are almost uniform as shown in FIG. 48(B) due to such workings of the lens arrays 21 and 22 and the condenser lens 23.
In the luminaire of FIG. 47, manufacture error in dimension associated with constituents and mounting error thereof are varied every device. Since a liquid crystal projector or the like is an mass-produced article, as shown in FIG. 48(A), the size of an illumination spot P on the plane 24 is necessary to be determined in consideration of a margin area MA provided outside an effective illumination area EA so as to absorb such errors. For example, in a case where the effective illumination area EA is a liquid crystal panel in rectangular shape with a longitudinal side of 15 mm and a lateral side of 20 mm, with the margin area MA having each width of 3.5 mm in the vertical direction and each width of 2.55 mm in the horizontal direction in order to cope with such errors as actually arise, 45% of the luminance light in amount is wasted in the margin area MA if considered a uniform distribution as shown in FIG. 48(B), resulting in poor light utilization efficiency.
Now, in an unpolarized light luminaire 10 shown in FIG. 49, the center of an arc gap of a metal halide lamp 11 is sat to the focal point of a parabolic mirror 12, and a UV/IR cut filter 13 is disposed on the aperture side of the parabolic mirror 12. Unpolarized light emitted from the metal halide lamp 11 is reflected by the parabolic mirror 12 to be substantially parallel light beams, and only white light passes through the UV/IR cut filter 13.
For example, in a liquid crystal projection device, light that has passed through a UV/IR cut filter 13 is directed to, as shown in FIG. 50, a lens array 21 of an polarization conversion device 20 in order to reduce heat absorption in a liquid crystal panel that uses only polarized light. A lens array 22 is of the same shape as that of the lens array 21 and arranged opposite to the lens array 21 with a distance A between the lens arrays 21 and 22, the distance being equal to a focal length of the lens array 21. A condenser lens 23 is disposed on the lens array 22 side and between the lens array 22 and a plane 24 to be illuminated. A focal length of the lens array 22 is not required to be equal to that of a lens array 21.
A parallel luminous flux having being incident on a lens 210 of the lens array 21 converges at the center of a corresponding lens 220 of the lens array 22 and is passes through the condenser lens 23 to project onto the whole plane 24 to be illuminated. Likewise, a parallel luminous flux having being incident on a lens 211 of the lens array 21 converges at the center of a corresponding lens 221 of the lens array 22 and is passes through the condenser lens 23 to project onto the whole plane 24. The emitting light from the unpolarized light luminaire 10 has an intensity distribution which is the highest in the middle and lower toward its periphery, but an almost flat illuminance distribution is obtained across an illumination spot on the plane 24 by such workings of the lens arrays 21 and 22 and the condenser lens 23.
A polarization conversion element 25 is disposed between the lens array 22 and the condenser lens 23.
The polarization conversion element 25 is constructed as follows. One end to the other end along X direction, arranging prisms 250 of the same shapes as one another, each having a section of a parallelogram and extending along a direction perpendicular to the drawing paper. Polarization beam splitters 251 and mirrors 252 each formed with dielectric multi-layered films are alternately inserted between the prisms 250 and formed on both end surfaces of the element facing X direction. Halfwave plates 253 are pasted on every other light emitting surfaces of the prisms 250. In this parallelogram, widths of a pair of an incident surface and an emitting surface opposite to each other is a half of a width CX of a lens of the lens array 22, lengths of the other pair of opposite sides are 2xe2x88x920.5CX, and one of opposite angle is 45 degrees. The polarization conversion elements 25 are arranged such that the centers of the lenses of the lens array 22 coincide with the centers in X direction of the polarization beam splitters 251.
For example, a p-polarized component of a light beam proceeding along the optical axis of a convex lens 211 is passes through a polarization beam splitter 251, while an s-polarized component thereof is reflected by the polarization beam splitter 251. The s-polarized component is further reflected by a mirror 252, and further passes through the halfwave plate 253 to be converted to p-polarized light. Therefore, light having passed through the polarization conversion element 25 is p-polarized light.
The metal halide lamp 11 in FIG. 49 has a light emitting part that is not a point source of light and therefore, emitted light from the unpolarized light luminaire 10 is not of perfect parallel beams. As shown in FIG. 50, most of non-parallel light beams, like a light beam L1, are reflected by the mirror 252 and s-polarized component of the reflected beams are further reflected by the polarization beam splitter 251 to be output from the polarization conversion element 25. Part of the non-parallel light beams, like a light beam L2, are passes through the polarization conversion element 25 with being kept in an unpolarized state. For these reasons, light utilization efficiency is reduced, which leads to darkness of a projected picture.
In FIG. 49, a divergence angle xcfx86 of the non-parallel beams whose intensity amount to more than half of that of parallel light beams is dependent on a visual angle from a point on the parabolic mirror 12 for the light emitting part. A parallelism of the emitting light beams from the unpolarized light luminaire 10 is reduced as the metal halide lamp 11 is of a higher power. For example, in order to obtain 1500 lm as a light power of the unpolarized luminaire 10, a metal halide lamp of more than 350 W is necessary, wherein a divergence angle xcfx86 of the non-parallel beams amounts to about 8 degrees at the maximum.
In FIG. 50, a visual angle xcex8 from a lens of the lens array 21 for the corresponding incident surface of the prism of the polarization conversion element 25 is expressed by the following formula.
xcex8=atan(CX/2A)xe2x80x83xe2x80x83(1)
Letting the length of the plane 24 be DX and the distance from the lens array 22 to the plane 24 be B, the following equation holds from the relation of geometrical similarity.
CX:DX=A:Bxe2x80x83xe2x80x83(2)
xcex8 is expressed as follow from the above two relations.
xcex8=atan (DX/2B)xe2x80x83xe2x80x83(3)
Namely, xcex8 is determined by values of DX and B.
It is required to satisfy a relation of xcfx86 less than xcex8 in order to improve light utilization efficiency.
In a case where a liquid crystal panel with a diagonal length of 50 mm is employed, since B and DX are usually of about 250 mm and 40 mm, respectively, xcex8 is of about 4.6 degrees. Therefore, in the case of the above described light source, light utilization efficiency is roughly half at the portion where xcfx86 is at the maximum, and the light utilization efficiency is of about 65% as a whole.
Now, FIG. 51 shows a schematic sectional view taken along the optical axis of a prior art polarized luminaire for use in a projection device. The arc gap center of a metal halide lamp 11 is set to the focal point of a parabolic mirror 12, and emitted light beams from the metal halide lamp 11 are reflected by the parabolic mirror 12 to be substantially parallel light beams. In a liquid crystal projection device, the parallel light beams are converted to linearly polarized light by a polarization conversion device 20X in order to improve light utilization efficiency at a liquid crystal panel which uses only linearly polarized light.
The polarization conversion device 20X is constructed of a prism 250X extending along a direction perpendicular to the drawing paper. The prism 250X has a section of a parallelogram and is joined with a right-angled triangular prism 255 with a polarization beam splitter 251X formed with dielectric multi-layered films interposed therebetween. A total reflection mirror 252X is formed on a surface of the prism 250X opposite to the polarization beam splitter 251X. A halfwave plate 253X is pasted on a light emitting surface of the prism 250X.
A p-polarized component of incident light on the prism 250X transmits through the polarization beam splitter 251X, and an s-polarized component thereof is reflected by the polarization beam splitter 251X. The s-polarized component is reflected by the total reflection mirror 252X and then passes through the halfwave plate 253X to be finally converted to p-polarized light. With such a process, emitted light from the polarization conversion device 20X becomes all p-polarized light.
In the polarization conversion device 20X, since a thickness D of the polarization conversion device 20X is equal to the aperture radius of the parabolic mirror 12 and a length of a light emitting surface thereof is equal to 4 times the aperture radius, there are problems due to its large size and heavy weight.
FIG. 52 shows a schematic sectional view taken along the optical axis of another prior art polarized luminaire for use in a projection device.
A polarization conversion element 25 of a polarization conversion device 20Y is constructed as follows. From one end to the other end along a direction perpendicular to the optical axis, prisms 250 of the same shapes as one another are arranged, each prism with a section of a parallelogram and extending along a direction perpendicular to the drawing paper. Polarization beam splitters 251 and total reflection mirrors 252 made of dielectric multi-layered films are alternately inserted between the prisms 250 and formed on both end surfaces of the element facing the direction perpendicular to the optical axis. Halfwave plates 253 are pasted on every other light emitting surfaces of the prisms 250.
The polarization conversion element 25 performs polarization conversion of incident light on every other prisms (light incident on a useful incident area) from unpolarized light to p-polarized light similar to the polarization conversion device 20X of FIG. 51, and converts incident light on the other prisms (light incident on a useless incident area) to s-polarized light. On the other hand, parallel light beams emitted from the parabolic mirror 12 are converged only to useful incident areas by the lens array 21, and p-polarized light beams emerged from the polarization conversion element 25 are collimated by the lens array 28.
According to such a polarization conversion device 20Y, a light emitting surface of the device is narrower than that of the polarization conversion device 20X of FIG. 51, enabling the device to be more compact and lighter.
Herein, in FIG. 49, the divergence angle xcfx86 of non-parallel light beams whose light intensity is more than half of the intensity of parallel light beams is, for example, 4 degrees. Even with this divergence, as in FIG. 52, since the light beams are converged on useful incident areas on the polarization conversion element 25 by the lens array 21, it can be prevented from occurring that light beams impinge on useless incident areas.
FIG. 53 is a optical path diagram showing a relation between a combination of convex lenses 211 and 281 in FIG. 52, and a divergence angle caused by a non-point light source.
Letting a focal length of the convex lens 211 be 2F, a focal length of the convex lens 281 is F so that a parallel light flux is converted to a parallel light flux of half the diameter, and a distance between the convex lenses 211 and 281 is 3F.
An image of the light emitting part of the metal halide lamp 11 is formed at a focal point of the convex lens 211. A visual angle from the center of the convex lens 211 for the image of the light emitting part is 2xcex8, and a visual angle from the center of the convex lens 281 for the image is 4xcex8.
Accordingly, a divergence angle is twofold owing to a light flux from the parabolic mirror 12 passing through the polarization conversion device 20Y. Therefore, if the polarized luminaire of FIG. 52 is employed in a liquid crystal projection device, a light amount that leaks outside a useful incident area increases at a liquid crystal panel and a projection lens, causing a darker projection picture. If a diameter of a projection lens is increased in order to improve light utilization efficiency, the device becomes heavier and more costly.
Accordingly, it is an object of the present invention to provide a luminaire with improved light utilization efficiency.
In the first aspect of the present invention, there is provided an illuminance distribution improving device for a luminaire comprising: a first lens array having a plurality of convex lenses each having a first focal length; a second lens array, having a plurality of lenses corresponding to respective lenses of the first lens array, disposed opposite to the first lens array; and a condenser lens disposed on the opposite side to the first lens array with respect to the second lens array, wherein a distance between the first and second lens arrays is substantially equal to the first focal length, wherein the second lens array has: a first lens group composed of a plurality of convex lenses each having a second focal length; and a second lens group composed of a plurality of lenses each having a second focal length negative or longer than that of the convex lenses of the first lens group.
In regard to the second lens array, assuming that only the first lens group is used, an illuminance distribution at a plane to be illuminated is substantially uniform with steep descent around it, for example as shown in FIG. 2(A). While, assuming that only the second lens group is used, an illuminance distribution at the plane to be illuminated has a mountain-like shape with its top in the center, for example as shown in FIG. 2(B). An actual illuminance distribution is obtained from superimposing both these two illuminance distributions, and for example as shown in FIG. 2(C), the illuminance almost smoothly changes within an effective illumination area, that is, a necessary illumination area EA.
Human eyes is not so sensitive for such a smoothly change of illuminance distribution that, if such a illuminance distribution improving device is used for a luminaire, for example, in a movie projector, and human eyes feel that the illuminated area is brighter, in a case where an illuminance in a central area is higher than that in a surrounding area thereof, than in a case of a uniform illuminance distribution, assumed that quantities of total light are same as each other. Furthermore, since an illuminance at the edge of the effective illumination area EA is lower than that at the center point and an illuminance steeply decreases from an edge point of the effective illumination area EA toward the outside, the ratio of light quantity into the margin area MA to the total light quantity is lower than that in the prior art if the same margin area is ensured.
Therefore, with the first aspect of the present invention, light utilization efficiency can be increased with almost no deterioration in image quality.
In the second aspect of the present invention, there is provided a polarization conversion device comprising: a first lens array having a plurality of convex lenses each having a first focal length; a second lens array, having a plurality of lenses corresponding to respective lenses of the first lens array, disposed opposite to the first lens array; a condenser lens disposed on the opposite side to the first lens array with respect to the second lens array; and a polarization conversion element, disposed between the second lens array and the condenser lens, wherein a distance between the first and second lens arrays is substantially equal to the first focal length, wherein the polarization conversion element comprises first and second polarization conversion elements, each of the first and second polarization conversion elements having a plurality of polarization conversion components arranged in parallel to one another, wherein each of the polarization conversion components has a band-shaped useful incident area and a band-shaped useless incident area adjacent to each other on a front surface thereof, each of the polarization conversion components is for converting unpolarized light incident on the useful incident area to a first linearly polarized light and for converting unpolarized light incident on the useless incident area to a second linearly polarized light having a polarization plane perpendicular to that of the first linearly polarized light or making incident light pass through without polarizing, wherein the band-shaped useful incident areas of the first polarization conversion components and the band-shaped useful incident areas of the second polarization conversion components are perpendicular to each other, wherein the first and second polarization conversion elements are arranged in such a way that central axes of the lenses of the second lens array cross corresponding central lines of the useful incident areas.
In the prior art, since longitudinal directions of band-shaped useful incident areas on a polarization conversion element are the same as one another, as shown in FIG. 27(B), in a place where light utilization efficiency is the poorest, although both end portions of the image of a light emitting part are the brightest, but are outside the useful incident area so that light impinged on both end portions are useless. In contrast to this, with the first aspect of the present invention, since the longitudinal direction of the second polarization conversion element corresponding to such a poor place is aligned with the same direction as that of images of a light emitting part, light utilization efficiency can be improved.
In the third aspect of the present invention, there is provided a polarization conversion device comprising first and second polarization conversion sections disposed adjacent to each other with optical axes of the first and second polarization conversion sections being substantially parallel to each other, wherein the first polarization conversion section comprises: a first polarization converter, having a plurality of first prisms arranged in a row, having polarization beam splitter films and reflection films alternately inserted between the first prisms without spacing, each first prism having a cross section of a first parallelogram, the first parallelogram having a pair of opposite angles of 45 degrees; a first lens array for converging incident light and directing converged light into every other first prisms of the first polarization converter; and a second lens array for collimating converged light from the fist prisms, wherein the second polarization conversion section comprises: a second polarization converter, having a second prism which has a cross section of a second parallelogram similar to and larger than the first parallelogram, having a polarization beam splitter film and a reflection film formed on opposite surfaces of the second prism, respectively.
With the third aspect of the present invention, although parallelism of light beams are deteriorated due to passage of the light beams through the first polarization conversion section which can be thinner, almost all of the light beams therefrom can be directed to a plane to be illuminated and thereby, light utilization efficiency is increased. Further, since the thickness of the second polarization conversion section can be smaller than in a case where only the second polarization conversion section is employed, the luminaire can be more compact and lighter.